Monolayer materials, such as graphene, are materials with great potential for electronics and other future carbon-based device architectures. Graphene is the two-dimensional (2D) form of crystalline carbon. It is a single atomic sheet of sp2-bonded carbon arranged in a honeycomb lattice extending in a single plane. As illustrated in FIGS. 1A-1D, graphene is the building block for the entire family of graphitic materials. For instance, graphene formed into a ball (see FIG. 1B) results in a carbon fullerene (buckyball); formed into a tube (see FIG. 1C) results in a carbon nanotube; and stacked at least ten layers high (see FIG. 1D), the graphene transforms into bulk graphite.
In fact, by stacking more and more graphene layers on top of each other, the material's properties change dramatically. A single layer of graphene exhibits a quantum staircase in Hall conductivity and ballistic transport, i.e., its charge carriers behave as massless Dirac fermions: charge carriers in the single layer can travel thousands of interatomic distances without scattering. Nano scale ribbons of graphene exhibit quantum confinement, and the capability for single-molecule gas detection. Graphene's physical properties are equally impressive. Measurements probing the intrinsic strength of a sheet of graphene reveal that it is the strongest known material. At two layers thick, graphene is still a zero-gap semiconductor exhibiting the quantum Hall effect. But, unlike single-layer graphene, double-layer graphene lacks a first “step” in the quantum staircase. For three or more graphene layers, however, the electronic properties begin to diverge, ultimately approaching the 3D limit of bulk carbon at about ten layers in thickness and more appropriately referred to as graphite.
One distinct advantage of graphene lies in its 2D nature, so that the drive current of a graphene device, in principle, can be easily scaled up by increasing the device channel width. This width scaling capability of graphene is of great significance for realizing high-frequency graphene devices with sufficient drive current for large circuits and associated measurements. Furthermore, the planar graphene allows for the fabrication of graphene devices and integrated circuits utilizing well-established planar processes in the semiconductor industry. A review of graphene is provided, for example, by A. K. Geim, et al. in “The Rise of Graphene,” Nature Materials 6, 183 (2007) and in “Graphene: Exploring Carbon Flatland,” Physics Today, 60, p. 35 (2007) each of which, along with the references cited therein, is incorporated by reference in its entirety as if fully set forth in this specification.
These remarkable properties make graphene suitable for a wide variety of applications. Potential applications in electronics include use of graphene as a new channel material for field-effect transistors (FETs) and as a conductive sheet in the fabrication of single-electron transistor (SET) circuitry. Another potential application is graphene-based composite materials in which a graphene powder is dispersed within a polymer matrix. Graphene powder may also find applications in batteries, as field emitters in plasma displays, or as a catalyst due to its extraordinarily high surface area. Single graphene sheets have exceptionally low-noise electronic characteristics, thereby lending the possibility of their use as probes capable of detecting minuscule changes in external charge, magnetic fields, or mechanical strain.
Despite the extraordinary potential of graphene, realization of practical applications which exploit its unique properties requires the development of reliable methods for fabricating large-area, single-crystal, and defect-free graphene domains. Recent attempts to produce monolayer and/or few-layer graphene have involved, for example, mechanical exfoliation of graphite crystals, thermal decomposition of silicon carbide (SiC) at elevated temperatures, reduction of graphene oxide in hydrazine, and epitaxial growth on transition metal surfaces. However, it continues to be a challenge to efficiently and reproducibly form large (>100 μm) single-crystal domains in quantities sufficient for large-scale fabrication.
For instance, chemical exfoliation involves inserting (“intercalating”) molecules into bulk graphite in order to separate the crystalline planes into individual graphene layers. The benefit of this technique is its facile chemical approach. The problem, however, is that even after the intercalating molecules are removed from the mixture, the resultant carbon compounds are present in a “sludge,” which contains both restacked and scrolled graphene sheets. (See M. S. Dresselhaus & G. Dresselhaus, Adv. Phys., 51, 1-186, (2002), incorporated herein by reference in its entirety.) Chemical epitaxy, on the other hand, offers the solution to graphene's large-scale integration challenge. In one version of the method, graphene is grown via chemical vapor deposition (CVD) of hydrocarbons deposited on a metal substrate. But, the presence (or remaining residue) of the metal substrate used in the CVD method might not be compatible with electronic fabrication. In contrast to the CVD method, the thermal decomposition method begins with a semiconducting SiC substrate, which is heated to over 1200° C. until the silicon begins to sublime, at which point the remaining carbon on top of the substrate nucleates into graphitic film. The resultant graphene/SiC sample can then be mounted on a silicon substrate for device integration. This thermal decomposition method can achieve few-layer graphene that exhibits high-mobility charge transport. This method, however, requires high-temperature vacuum processing. Consequently, the formation of graphene domains with uniform thicknesses and length scales sufficient for practical applications remains a challenge. (See C. Berger et al., J. Phys. Chem. B 108, pp. 19912-19916, (2004), incorporated herein by reference in its entirety.) One approach to epitaxially grow the graphene on the ruthenium (Ru) transition metal that avoids the shortcomings noted above is described in U.S. Pat. Pub. No. 2010/0255984 to Sutter et al.
However, while epitaxial growth on transition metal surfaces is key to realizing large-scale graphene growth, forming conventional and spin-polarizing device contacts, and accessing functionalities such as magnetism and superconductivity, as well as having important implications for transition-metal surface chemistry and catalysis in the presence of graphitic carbon, the method also results in a strong interfacial interaction of transition metal with graphene that suppresses the characteristic linear π bands of its electronic structure. This suppression hinders the rise of the high-mobility massless Dirac quasi-particles.
Efforts to change the graphene-transition metal interaction have largely focused on intercalation of metal atoms and, recently, hydrogen (For example, see Varykhalov, A. et al., Phys. Rev. Lett., 101, p. 157601 (2008); Oshima, C. and Nagashima, A., J. Phys: Condens. Matter, 9, pp. 1-20 (1997); Nagashima, A. et al., Phys. Rev. B, 50, pp. 17487-17495 (1994); and Biedl, C. et al., Phys. Rev. Lett., 103 p. 246804 (2009); each incorporated herein by reference in its entirety.)
Thus, despite the extraordinary potential of graphene, realization of practical applications that exploit its unique properties requires the development of reliable methods for fabricating large-area, single-crystal, and defect-free graphene domains that can be effectively lifted off the metal substrate despite a strong metal-carbon coupling and thereby restore the characteristic linear π bands that give rise to high-mobility massless Dirac quasi-particles in the monolayer graphene.